This application claims priority under 35 U.S.C. xc2xa7xc2xa7119 and/or 365 to Appln. No. 199 57 276.3 filed in Germany on Nov. 29, 1999; the entire content of which is hereby incorporated by reference.
The present invention relates to addition-crosslinking silicone rubber mixtures which display, in particular, very good adhesion to numerous substrates and a considerably increased reaction rate in the crosslinking process. Addition-crosslinking silicone rubber mixtures according to the invention are used, in particular, as coatings on substrates, for example on glass, ceramic, metals or plastics, or in the production of composites.
Liquid addition-crosslinking silicone rubber mixtures comprising (i) at least one organopolysiloxane containing alkenyl groups and (ii) at least one organohydrogenpolysiloxane are known per se. To accelerate the crosslinking addition reaction between the alkenyl group (xe2x80x94CHxe2x95x90CH2) and the Hxe2x80x94Si group, such a mixture generally further comprises, as catalyst, at least one known complex of a metal selected from the group consisting of rhodium, nickel, palladium and platinum, preferably complexes of platinum. However, it is also known that the cured solid silicone rubbers obtained from the abovementioned liquid to paste-like mixtures have unsatisfactory adhesion to substrates such as glass, ceramic, metal or plastic. Attempts have therefore been made to improve these adhesion properties by addition of additives so that silicone rubbers of this type can be used for producing coatings and composites.
U.S. Pat. No. 4,087,585 proposes the addition of a short-chain polysiloxane containing at least one SiOH group and a silane containing at least one epoxide group and an Si-bonded alkoxy group to achieve satisfactory adhesion to aluminum. According to EP-A-0 326 712, improved adhesion to various plastics is achieved by means of a mixture or a reaction product of (a) a silicon-free compound containing at least one alcoholic OH group and at least one alkylene group and (b) an organosilane containing at least one alkoxy group and at least one epoxide group, but this requires relatively long reaction times (1 hour) at a temperature of 120xc2x0 C. The long reaction times are frequently caused by the adhesion promoters which are simultaneously present and have an inhibiting action. According to EP-A-0 503 975, inhibition by such additives can be improved, but only to a limited extent, by selection of an optimized ratio of the unsaturated CH2xe2x95x90CHxe2x80x94Sixe2x80x94 groups to the SiH groups. Improved adhesion to aluminum is, for example, achieved only after a relatively long vulcanization time of 2 hours at 100xc2x0 C. Possible shortening of the reaction times by increasing the temperature is not possible in the case of, in particular, many polymer substrates because of their lack of heat resistance.
EP-A-0 875 536 proposes the combination of organopolysiloxanes containing alkenyl groups with selected silanes and at least one alkoxysilane containing at least one epoxide group and/or an alkoxysilane containing at least one epoxide group.
Accordingly, one object of the invention is to provide novel addition-crosslinking silicone rubber mixtures which display good adhesion to substrates and do not have the disadvantages of the prior art, e.g. low reactivity or a multiplicity of additives.
It has now been found that addition-crosslinking silicone rubber mixtures, preferably as liquid to paste-like mixtures, comprising at least one organopolysiloxane containing alkenyl groups and at least one organohydrogenpolysiloxane which may contain dimethylsilyloxy groups and, as catalyst, cis-dichlorobis(styrene)platinum(II), if desired in admixture with further customary catalysts, without additives, give a high reactivity and a cured product having very good adhesion properties. The addition-crosslinking silicone rubber mixtures of the invention can further comprise the customary additional constituents known per se. The preparation of cis-dichlorobis (styrene)platinum(II) is known per se and is described, for example, in W. R. Caseri, thesis, ETH Zurich, 1988. It is surprising that cis-dichlorobis (styrene)platinum(II) as selected platinum catalyst not only promotes the addition reaction but also considerably improves, compared to other catalysts known per se, the adhesion properties of the cured product. A composition comprising the components (a), (b) and (c) and optionally (d) defined below, where the molar ratio of the SiH groups present in the mixture to the Si-bonded alkenyl groups present in the mixture is at least 1.5, preferably from about 1.5 to 4.5 and more preferably from about 1.8 to about 2.5, has not previously been described in connection with the excellent adhesion properties associated therewith. Particularly good results are obtained when the surface to be coated is treated prior to the coating procedure with a degreasing agent which has a slightly corrosive action, as is described below for the process for the use of the silicone rubber mixture of the invention.